| Isopoda | |
|---|---|
 | |
| Eurydice pulchra, a carnivorous isopod found on sandy shores | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Subphylum: | Crustacea |
| Class: | Malacostraca |
| Superorder: | Peracarida |
| Order: | Isopoda Latreille, 1817 [1] |
| Suborders | |
Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax.
Isopods have various feeding methods: some eat dead or decaying plant and animal matter, others are grazers, or filter feeders, a few are predators, and some are internal or external parasites, mostly of fish. Aquatic speciesmostly live on the seabed or bottom of freshwater bodies of water, but some taxa can swim for a short distance. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball as a defense mechanism or to conserve moisture.
There are over 10,000 species of isopod worldwide, with around 4,500 species found in marine environments, mostly on the seabed, 500 species in fresh water, and another 5,000 species on land. The order is divided into eleven suborders. The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago, when isopods lived in shallow seas. The name Isopoda is derived from the Greek roots iso- (from ἴσος ísos, meaning "equal") and -pod (from ποδ-, the stem of πούς poús, meaning "foot").[2][3]
The World Marine, Freshwater and Terrestrial Isopod Crustaceans database subdivides the order into eleven suborders:[1]
- Asellota – This suborder contains the superfamily Aselloidea, a group that contains most of the freshwater isopods in the northern hemisphere, and the superfamilies Stenetrioidea, Gnathostenetroidoidea and Janiroidea, which are mostly marine. The latter superfamily, Janiroidea, has a massive radiation of deepsea families, many which have taken bizarre forms.
- Calabozoida – A small suborder consisting of two marine species in the family Calabozoidae and one freshwater species in the family Brasileirinidae which is found in subterranean locations.[18]
- Cymothoida – Chiefly marine isopods[9] with over 2,700 species. Members are mostly carnivorous or parasitic. Includes the family Gnathiidae, the juveniles of which are parasitic on fishes.[19] The previously recognised suborder Epicaridea is included as two superfamilies within this suborder and Cymothoida now includes part of the formerly recognised suborder Flabellifera.[20] Also includes the former suborder Anthuridea, a group of worm-like isopods with very long bodies.
- Limnoriidea – Mainly tropical isopods, some of which are herbivorous.[20]
- Microcerberidea – Tiny, worm-like isopods that live between particles on the bed of freshwater and shallow marine habitats.[9]
- Oniscidea – Semi-terrestrial and terrestrial isopods fully adapted for life on land.[9] There are over 4,000 species of woodlice inhabiting forests, mountains, deserts and the littoral zone.[21]
- Phoratopidea – A single marine species, Phoratopus remex, which warrants its own suborder because of its unique characteristics.[20]
- Phreatoicidea – Small suborder of freshwater isopods resembling amphipods, limited to South Africa, India, Australia and New Zealand.[9]
- Sphaeromatidea – Benthic isopods mostly from the southern hemisphere with respiratory pleopods inside a branchial chamber. This suborder now includes part of the formerly recognised suborder Flabellifera.[20][22]
- Tainisopidea – Freshwater isopods in a "relictual environment".[20]
- Valvifera – A large group of benthic, marine isopods with respiratory pleopods inside a branchial chamber under the abdomen.[9]
Unlike the amphipods, marine and freshwater isopods are entirely benthic. This gives them little chance to disperse to new regions and may explain why so many species are endemic to restricted ranges. Crawling is the primary means of locomotion, and some species bore into the seabed, the ground or timber structures. Some members of the Flabellifera can swim to a limited extent and have their front three pairs of pleopods modified for this purpose, with their respiratory structures limited to the hind pleopods. Most terrestrial species are slow-moving and conceal themselves under objects or hide in crevices or under bark. The semi-terrestrial sea slaters (Ligia spp.) can run rapidly on land and many terrestrial species can roll themselves into a ball when threatened, a feature that has evolved independently in different groups and also in the marine sphaeromatids.[9]
Isopods first appeared in the fossil record during the Carboniferous period of the Paleozoic some 300 million years ago.[23] They were primitive, short-tailed members of the suborder Phreatoicidea. At that time, Phreatoicideans were marine organisms with a cosmopolitan distribution. Nowadays, the members of this formerly widespread suborder form relic populations in freshwater environments in South Africa, India and Oceania, the greatest number of species being in Tasmania. Other primitive, short-tailed suborders include Asellota, Microcerberidea, Calabozoidea and the terrestrial Oniscidea.[14]
The short-tailed isopods have a short pleotelson and terminal, stylus-like uropods and have a sedentary lifestyle on or under the sediment on the seabed. The long-tailed isopods have a long pleotelson and broad lateral uropods which can be used in swimming. They are much more active and can launch themselves off the seabed and swim for short distances. The more advanced long-tailed isopods are mostly endemic to the southern hemisphere and may have radiated on the ancient supercontinent of Gondwana soon after it broke away from Laurasia 200 million years ago. The short-tailed forms may have been driven from the shallow seas in which they lived by increased predatory pressure from marine fish, their main predators. The development of the long-tailed forms may also have provided competition that helped force the short-tailed forms into refugia. The latter are now restricted to environments such as the deep sea, freshwater, groundwater and dry land. Isopods in the suborder Asellota are by far the most species-rich group of deep sea isopods.[14]
The majority of crustaceans are aquatic and the isopods are one of the few groups of which some members now live on land.[29][30] The only other crustaceans which include a small number of terrestrial species are amphipods (like sandhoppers) and decapods (crabs, shrimp, etc.).[29] Terrestrial isopods play an important role in many tropical and temperate ecosystems by aiding in the decomposition of plant material through mechanical and chemical means, and by enhancing the activity of microbes.[31] Macro-detritivores, including terrestrial isopods, are absent from arctic and sub-arctic regions, but have the potential to expand their range with increased temperatures in high latitudes.[32]
The woodlice, suborder Oniscidea, are the most successful group of terrestrial crustaceans[9] and show various adaptations for life on land. They are subject to evaporation, especially from their ventral area, and as they do not have a waxy cuticle, they need to conserve water, often living in a humid environment and sheltering under stones, bark, debris or leaf litter. Desert species are usually nocturnal, spending the day in a burrow and emerging at night. Moisture is achieved through food sources or by drinking, and some species can form their paired uropodal appendages into a tube and funnel water from dewdrops onto their pleopods. In many taxa, the respiratory structures on the endopods are internal, with a spiracle and pseudotrachaea, which resemble lungs. In others, the endopod is folded inside the adjoining exopod (outer branch of the pleopod). Both these arrangements help to prevent evaporation from the respiratory surfaces.[9]
Many species can roll themselves into a ball, a behaviour used in defence that also conserves moisture. Members of the families Ligiidae and Tylidae, commonly known as rock lice or sea slaters, are the least specialised of the woodlice for life on land. They inhabit the splash zone on rocky shores, jetties and pilings, may hide under debris washed up on the shore and can swim if immersed in water.[9]
https://en.wikipedia.org/wiki/Isopoda
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